(CNN) -- Peter Weir doesn't make films in a hurry -- just 10 over the past 30 years, and nothing at all since "Master and Commander" in 2003 -- but they're almost always worth waiting for. Among them, "Witness" stands as a popular classic, while "Picnic at Hanging Rock" remains a landmark in Australian cinema, an enigma throbbing with atmosphere.

"The Way Back" is a full-scale epic: a World War II era survival story about six men who break out of a Siberian gulag and their perilous 4,000-mile walk to freedom. It's an ambitious, imposing film in which nature supplies the special effects, but at the same time, audiences may find it's a long slog and a bit of an endurance test.

The tempo is, well, walking pace.

Not that Weir wastes much time in the gulag itself, a hellhole in which the prisoners fight for scraps of food, the barracks are overrun with lice and labor details are left to freeze in blizzards or work in the mines until they drop. The escape itself is easily accomplished, a break in the barbed wire fence when a snowstorm has blinded the guards.

If security is lax, it's because the frying pan may be worse than the fire. Siberia in the depths of winter offers infinite forests, subarctic temperatures, and minimal sustenance. It takes a special kind of desperation to tackle those odds.

Six men make the break, united only in a plan to head south and keep going until they have walked their way out of Stalin's reach.

Janusz (Jim Sturgess) is a Polish political prisoner haunted by the memory of his betrayal by his wife. A taciturn American, Mr. Smith (Ed Harris), is convinced that they will perish in the mines within months. Valka, a Russian gangster (Colin Farrell), has racked up too many debts inside and he's certain he'll be murdered if he doesn't get out.

It's a true story -- maybe. Slavomir Rawicz's 1954 memoir "The Long Walk" has been challenged by historians, and it may be safest to treat his account as historical fiction. For better or worse, Weir and co-writer Keith Clarke keep the phony dramatics to a minimum. There's not even much talk, until Saoirse Ronan's teenage runaway hooks up with them and gradually unlocks the men's stories.

The emphasis is on grueling physical hardship, ice, snow and sandstorms as well as the real possibility of starvation, and the will that keeps these men trudging forward into Mongolia, across the Gobi Desert, into Tibet and ultimately across the Himalayas.

From the artificial TV world of "The Truman Show" to the jungle in "The Mosquito Coast," Weir has always been particularly sensitive to how environment affects the human soul. Here that theme gets a broad canvas, and while not all the men make it, it's a surprisingly optimistic film in which Janusz not only retains his compassion, but wins over Farrell's callous thug and Harris' withdrawn American. Ironically, while the characters suffer, we are privileged to bathe in Russell Boyd's glorious cinematography.

If the film doesn't register with the impact of David Lean's epics -- and watching it, we're bound to think of "The Bridge on the River Kwai" and "Lawrence of Arabia" -- it's because there's simply not enough dramatic variety, no relief from the relentless grind of putting one foot in front of the other.

Weir must have sensed this, too: The last stage of the odyssey, a trek through the Himalayas, is tossed off in a couple of minutes, as if it were a stroll in the park. Most audiences will appreciate that brevity, I fancy, even if readers of Rawicz's memoir will miss the mention of two mysterious yeti-like creatures. Perhaps they'll make it to the director's cut?